A painful memory has a stubborn grip. That is the central idea here: happy moments slip away almost as soon as they happen, but hurtful moments return and cling. The speaker sounds quietly weary, as if she has learned through experience that joy is fragile and temporary, while pain has a way of digging in. The image of happy hours fading “like shadows in a looking-glass” makes happiness feel ghostly and unreal—something you can see for a moment but cannot hold. By contrast, a stinging memory is described as the thing that “clings” most in the world, which gives the poem a melancholy, resigned mood. It does not rage against this unfairness; it simply states it as an inevitable truth, leaving the reader with a faint, bittersweet ache.
Inevitably — Summary
There's nothing in the world that clings As does a memory that stings; While happy hours fade and pass, Like shadows in a looking-glass.